How to paint car body panels

So I'm going to assume you need to do a serious repair, stripping part of the panel back to bare metal and panel-beating it back into shape.

I wasn't actually looking to make a perfect job of my bent car wing (fender); I just wanted to see if I could rescue the panel instead of buying a new one (which would have been so much simpler). I tried to find a second hand wing, but as it happens my car is some sort of special edition (I didn't even know!) and the wing is a rare type without an indicator light hole. So the only place I can get one is new from the main dealer.

Panel beating

You will most likely need a set of panel beating hammers and dollies. These can be picked up relatively cheaply from ebay. I paid about £20.

The first small mistake I made was filing and polishing some of the hammer faces perfectly and shiny flat. They came with a fairly rough filed texture with a slight convex shape. I figured this wasn't good because every time I hit the panel it would be trying to create more dents rather than flattening it out.

My work wasn't a complete waste of time, I do think it helped, but the main mistake was putting fairly sharp edges on the hammer faces. If you got a hit slightly wrong you could easily put extra dents and creases in the panel.

If I did it again I would still make them flat and polished, but would put a radius on the edges to avoid severe creases if you get it wrong. Or just file and polish the centre of the face flat, depending on what you get.

First I would concentrate on getting the panel roughly into the right shape and concentrate on getting the edges and and panel creases looking right. Check with the same panel from the other side of the car and mimic as best you can. I think the idea is to work around a dent from the edges and into the centre.

Pick the most appropriate dollie, put it behind the work and hammer into the dollie through the panel. As it happened I didn't make too bad a job. The creases were pretty good and I ended up with two 'lumps' about 2-3 cms (1") in diameter that I figured I could fill around or even high-build prime away. More on that later.

The other thing I will note about panel beating is that, I think once the metal is stretched, it is probably impossible to get it to unstretch. In other words the sheet has become thinner in places, and it will be impossible to get it thicker by using a hammer. The best you can hope for is either:

1. Spread the stretch out into a bigger area so any dent is larger but less prominent, or;
2. Dent it inward ready for filling

The other thing I heard is that heat helps restore the panel back to its original shape (it has a memory) and is helpful for stopping the paint from cracking causing more paint work. I only tried this briefly with a hot air gun so don't know if it really helps. It is difficult to both heat a large panel up to a decent temperature as well as work on it. There are likely special heaters for this.

Types of paint

Next is to figure out what sort of paint to use. I found out the following types:

1. Epoxy primer: if you have gone back to bare metal it looks like this is essential to seal the metal again. Other types of paint like acrylic primer are not waterproof (are porous) so once the bare metal is stripped back and sanded, this is the only real option to make it rustproof again. Normally epoxy comes as 2K (2-part) and needs to be mixed in a professional paint gun. I managed to find some 1K (1 part) spray can stuff from BuzzWeld (here in the UK) on ebay. It has a year shelf-life because the hardener is already mixed in, but I think is inhibited with another solvent. So once it is out of the can it starts to cure. Mine was black and seemed quite thick, so was actually useful to put several coats on to start filling the shallow dents and scratches and blend everything together. You want to make this as thick as possible so that if you sand back to this primer, you don't blow straight back to metal. Take care to put lots on the panel edges because they are the easiest parts to over-sand and blow through. Yes I did this several times in small areas. So make sure you leave some epoxy in the can and always patch up the bare metal;
2. Acrylic primer comes as normal and high-build (thicker) types. This is pretty common stuff, often in a light grey colour available from most car parts shops. As said above, it is not waterproof so only really useful as a sandwich layer before the topcoat. I used this pretty effectively to get rid of the main bumps and dents instead of filling. More on that later;
3. Enamel paints are also very common in stores that sell rattle cans. From my research these are not really supposed to be used on car panels and are more of a general purpose, low-cost metal paint;
4. I also had some PlastiKote combined metal primer and paint. Apparently this is also not supposed to be used on car panels because it has a different formulation that can react with other types of paint. So again, only really useful as a general purpose, one layer metal spray paint. I used mine on one of my windscreen wipers, which was aluminium, had been chipped badly and was corroding. It made a really good job and is still fine today;
5. Top coat and clear coats are normally a 2K paint mixed in the gun. I will get mine sprayed by a local professional but you can buy / get tins made up with the colour you need;
6. Rust converter - I used some rust converter in a rush to provide some protection after my initial go at panel beating and sanding. Whilst I think it worked to stop most of the rust over several months it really wasn't a good move. I should have got the epoxy primer in advance.

My biggest tip / mistake!

DO NOT use Acetone to clean your paintwork!

I bought some acetone to remove the rust converter I had previously applied. If you can't find any, check your local health / cosmetic store (Superdrug in the UK) for nail varnish remover. Whilst it did a great job of removing the rust converter, I did notice it would quickly remove other layers of paint from the underside of the panel very quickly. This should have been my clue. A lot of spray paints have acetone as a solvent it turns out. That means they will easily dissolve in it!

So yes, it is very aggressive and will redissolve your work, even the epoxy primer! So don't do what I did and because it is lying around, use it to wipe down your work after sanding. What it will do is cause your dried paint to become 'wet' again. Then if you spray over the 'wet' paint you will most likely get crazing as the two paint layers have different levels of solvent and are at different cure stages.

It only really affected one small patch but I suspect will come back to bite me later. I sanded it back as best I could and covered over with high-build acrylic once dry but I think the damage is done.

Instead use isopropyl alcohol. It is pretty easy and cheap to get from ebay or hardware stores, is an excellent general purpose solvent for cleaning things and doesn't evaporate immediately unlike acetone. Most importantly, IPA doesn't seem to dissolve paint!

Grit sandpaper

What grit sandpaper should I use?

Sandpaper is used for providing a 'key' for the paint to grip onto. Don't do what i did a while ago and think you should get the surface perfectly smooth before painting. I have some 1000 - 3000 grit sandpapers, but sprays like acrylic paint will not stick to a surface prepared with those. So the perfect glass acrylic finish I got on a pair of metal headlights for my long-deceased Lotus Elise soon flaked off!

1. 60 - 150 grit for the bare metal and surrounding paintwork to give a key for the epoxy primer;
2. 150 - 240 grit for the epoxy and surrounding paintwork for the acrylic primer;
3. 240 - 400 grit for more layers of primer;
4. 400 - 600 grit for top coat prep;
5. Clear coat should be applied directly to the top coat whilst it is still in its recoat window.

Fill or layers of high-build primer?

Originally I was going to use Isopon P.38 filler to smooth out the lumps and bumps. But once I had the epoxy on and saw it was already filling some of the bumps after a sand, I decided to try to layer on the high-build acrylic primer. I only needed it in a fairly small area, so figured a whole tin would easily give me 10 to 20 thick coats. The plan would have worked without the acetone problem!

I still will not use filler and the result, I think, will look just a bit wobbly on close inspection, but that's good enough for me.